Sequestration of Halides from FGD Wastewater through Lime Stabilization of Fly Ash

Shubhashini Oza1, Vincent Ogunro1, Kirk M. Ellison2 and John Daniels1 1University of North Carolina at Charlotte, Dept. of Civil & Environmental Engineering, 9201 University City Blvd, Charlotte, NC 28262; 2Southern Company Services, 600 North 18th Street/14N-8195, Birmingham, Alabama 35203. KEYWORDS: halides, sequestration, flue gas desulfurization (FGD) wastewater (WW), lime stabilization ABSTRACT The U.S. Environmental Protection Agency released the “Mercury and Air Toxic Standards (MATS)” for power plants in December 2011 to limit mercury, acid gases, and other pollutants from power plants. As a consequence, new methods and technologies were developed to control air emission which augmented the challenges of dealing with the flue gas desulfurization wastewater (FGD WW) (to remove sulfur di-oxides). Typical FGD WW is high in halides (Cl-, Br- and SO4

2-) that cannot be cost-effectively treated and disposed of in water systems. Therefore, the goal of this study was to assess solidification/stabilization (SS) treatment option of sequestering the potential contaminants of concern (COC) of FGD WW in quicklime-treated fly ash. Class F fly ash for the study was obtained from a coal fired power plant in the southeastern part of the United States while the FGD WW was artificially synthesized in the lab. Initial characterization of the ash was conducted to determine physical and chemical properties, such as specific gravity, optimum moisture content, maximum dry density, and particle size distribution, x-ray fluorescence analysis, and IC and ICP-MS analysis. Fly ash was stabilized with varying lime contents and FGD WW. Unconfined compression tests were performed on the monoliths to determine any observable strength increase with respect to lime content. The leaching characteristics of these monoliths were studied as per U.S. LEAF Method1315.The results indicated a direct correlation between lime content and percentage leaching of halides. INTRODUCTION Typical FGD purge streams have very high concentrations of halides along with significant amount of sulfates, carbonates, alkali and alkaline metals. Successive regulation have lowered maximum allowable COC in the air emission and water discharge operations of industrial plants. With the proposed EPA’s “Steam Electric Power Generating Effluent Guidelines”, management of these COC and likely replacement to a cleaner zero liquid discharge (ZLD) will become increasing challenging. Therefore, sequestering contaminants in the solid phase remains an

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attractive and necessary option. Although, the ultimate goal of this project is to permanently sequester the COC from FGD wastewater streams, the specific objective for the discussion here is, to investigate the durability and the leaching mechanisms the monolith samples prepared from mix proportion for sequestering FGD waste water from ZLD processes. There are various encapsulation, solidification or sequestration studies which were evaluated (Table 1) for this study. Based on the literature, ongoing work, cost and material availability, the three priority technologies are modified form of Poz-o-Tec, geopolymers and Organosilanes. For this paper, the discussion is limited to the modified form of Poz-o-Tec technology. Table 1: List of various encapsulation/ solidification technologies available

Sl. No.

Technologies

1 Hydraulic Cement Solidification (HCS) 1 2 Environmental Technology Corporation (ETC) 2 3 Sulfur Polymer Stabilization/Solidification (SPSS) 3 4 Chemically Bonded Phosphate Ceramic Encapsulation (CBPS) 3b, 4 5 Polyethylene Encapsulation (PE) 3b, 5 6 Silicon-Polymer System Using Polysiloxane or Ceramic Silicon Foam (CSF) 6 7 Polyester Resin 7 8 Geopolymer or Geopolytech Process 8 9 Dolocrete Stabilization 4 10 Chemfix 2, 9 11 Poz-o-Tec 2 12 Organosilane Chemistry 10 13 TEGRATUFF 11

Poz-o-Tec is a process developed in the 1970s by IU Conversion Systems, Inc. to stabilize FGD sludge using lime, fly ash and other additives. Processing steps include dewatering of the sludge; addition of fly ash; the addition of lime to form cementitious material, and finally compaction of the mixture to increase strength and reduce permeability. The chemistry of reaction involves initial rapid reaction between soluble salts in the fly ash, lime, and alumina in the fly ash followed by a slower reaction between the silica in the fly ash 2. The key component is lime which is responsible for the several hydration reactions that occur. The hydration products formed from this reaction include mineral phases such as calcium silicate hydrates and ettringite. The ettringite family of mineral phases such as charlesite, sturmanite, and jouravskite are responsible for immobilization of chlorides, metals, and oxyanion forming elements. It was determined that ettringite formation was the primary binding mechanism for the materials, followed to varying degrees by calcium silicate hydrate (the principle binder in Portland cement). These minerals are responsible for high strength and reduced leachability. This process is applicable to all calcium based scrubbing system waste and

wastes containing salts/ heavy metals. According to Iadevaia2, Poz-o-Tec stabilized materials have compressive strength similar to low strength concrete. This study was investigates the efficiency of form of Poz-o-tec technology in sequestration of halides from FGD wastewater in a design mix, containing fly ash, lime and FGD wastewater. Since the technology adopted is a modified version of Poz-o-Tec, it would be addressed as Pseudo Poz-o-Tec technology from here on. The EPA Method 131512 was employed to assess the leaching and immobilization of halides and COC in the Pseudo Poz-o-Tec mix design. EXPERIMENTAL Materials The fly ash used in this study was obtained from a coal fired power plant in the southeastern part of the United States, FGD WW was synthesized in the lab and the quicklime was from Lhoist North America. The fly ash physical properties such as specific gravity (2.24, measured as per ASTM D854-1413), optimum moisture content (16.8%), maximum dry density (96.75 pcf), and particle size distribution (D90 is 83.82 µm) was measured. The fly ash chemical composition (Table 2) was determined by Wavelength Dispersive X-Ray Fluorescence (WDXRF) by RTI Labs, Raleigh, NC and the fly ash can be categorized as a class F fly ash according to the American Society for Testing and Materials (ASTM C618-12a) specification14. The chemical composition of the as-received FGD wastewater was analyzed using Ion Chromatography and Inductively Coupled Plasma – Mass Spectroscopy (Table 3). Table 2: Chemical composition of the fly ash (wt. %) Al2O3 BaO CaO Fe2O3 K2O MgO Na2O P2O5 SiO2 SrO Fly ash 28.65 0.07 1.94 6.99 0.8 1.49 1.71 0.45 51.92 0.1 Table 3: Chemical composition of the FGD wastewater Anion F- Cl- Br- NO3

- PO43- SO4

2- ppm ppm ppm ppm ppm ppm < 0.125 86,039 2,666 77 < 0.125 3,730

Metal B Cr Mn Ni Zinc As Se Mo Cd µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L 146,618 862 138 15 2,121 804 31,369 12 5,581 Pseduo Poz-o-Tec Mix Designs Before the mix design was formulated, the minimum lime requirement for the fly ash in discussion was verified by conducting the Eades and Grimm’s pH test as per ASTM

6276-99a15. The quicklime content ranged from 0 to 12% dry mass of the fly ash and the results indicated the ideal lime content in the range of 5 to 9%. Table 4 presents the mix design details. Table 4: Mix proportions for the Pseudo Poz-o-Tec mixes Mix Design Fly ash Lime Wastewater Total % % % % 5 % lime 67.00 5.00 28.00 100.00 6 % lime 66.00 6.00 28.00 100.00 7 % lime 65.00 7.00 28.00 100.00 8 % lime 64.00 8.00 28.00 100.00 9 % lime 63.00 9.00 28.00 100.00 Leaching Test In this study, EPA Method 1315 was considered to determine the leaching behavior of the stabilized mix designs. Method 1315 is a semi-dynamic tank leaching test performed on monolithic or compacted granular materials, and is used to determine the short and long term performance of the solidified matrix 12. In this test method, the sample (cured for 28 days at room temperature) is completely immersed in a designated laboratory-grade container filled with deionized water which is periodically renewed at nine selected leaching extraction intervals in days (d) (0.08 d, 1 d, 2 d, 7 d, 14 d, 28 d, 42 d, 49 d, and 63 d) (Figure 1 and 2). The leachate from all the 9 time intervals were filtered (0.45μm filter paper) and analyzed for pH, ORP, conductivity and concentration of anions (F-, Cl-, Br-, NO3

-, PO43- and SO4

2-) and metals (As, B, Ca, Cr, Cu, Fe, K, Mn Mg, Ni, Pb, Se, Na, Tl, V and Zn).

Figure 1: Schematic for Method 1315

Figure 2: Tank leaching Test (the lid of the tank is always closed during testing) Unconfined Compression (UC) Testing The unconfined compressive (UC) strength of the monoliths were assessed as per ASTM D216616. The compressive strength was measured at 7, 14, and 28 days of curing. Further, the UC strength was also measured post leaching (after 63 days - EPA Method 1315). RESULTS and DISCUSSION Leaching Test Although, 6 anions and 13 (B, V, Cr, Mn, Ni, Zn, As, Se, Mo, Cd, Sb, Tl, Pb) metals, where analyzed, not all these analytes were at detectable levels. Phosphate, nitrate and fluoride were at minimum detection limit (MLD) in the leachates for all the 5 mix designs. Among the 13 elements analyzed, only 5 (B, Mo, Se, V, Zn) of the elements were above the minimum detection range in the leachates. Table 5 presents the minimum detection limit for the 6 anions and 13 metals. The cumulative mass release was calculated and plotted for 3 anion and 5 elements only. The 0.5 slope line in the cumulative mass release aids in predicting the dominant release mechanism of COC at a particular extraction time interval. There are three dominant release mechanisms based on the slope value:

< 0.35 represents leaching due to surface depletion 0.35 to 0.65 represents leaching due to diffusion from the monoliths 0.65 represents leaching due to wash-off/dissolution from the monoliths (Time

interval T02-T01 is usually wash-off and thereafter would be dissolution) Chloride: leaching of chloride was observed at all-time intervals, and was

predominantly through diffusion mechanism. Based on slope for time intervals, lime content does not have significant impact on chloride sequestration (Figure 3). But with increase in lime content in the mix design, the leaching at each time interval is

Table 5: Minimum detection limit for anions and metals measured

Anions - Metals Minimum Detection Limit Units Fluoride 0.125 ppm Chloride 0.3125 ppm Bromide 0.125 ppm Nitrate 0.125 ppm Phosphate 0.125 ppm Sulfate 1.25 ppm Boron 1.00 ppb Vanadium 1.00 ppb Chromium 1.00 ppb Manganese 1.00 ppb Nickel 1.00 ppb Zinc 1.00 ppb Arsenic 1.00 ppb Selenium 1.00 ppb Molybdenum 1.00 ppb Cadmium 1.00 ppb Antimony 1.00 ppb Thallium 1.00 ppb Lead 1.00 ppb reduced most probably due to low hydraulic conductivity with increase in lime

content. Bromide: leaching is predominant for the first 5 time intervals and is mostly

dissolution and diffusion, but reduces at the end of leaching period. Increase in lime content results in marginal reduction in the amount of the element leached (Figure 4).

Boron: all mix designs indicated leaching with dissolution mechanism at all lime

contents (Figure 5). Increase in lime content in the mix-designs had marginally improvement the sequestration of the amount of the element.

Molybdenum: all the mix designs indicated dissolution leaching behavior. At higher

lime content, leaching was not observed at initial time intervals indicating that increase in lime content do significantly impact leaching of this element by delaying leachability (Figure 6).

Selenium: both dissolution (early time intervals) and diffusion leaching mechanism

was observed. Increase in lime content had marginal improvement on leaching (Figure 7).

Vanadium: no leaching was observed during the first 3 time intervals, but after that it was predominantly dissolution. Increase in lime content had marginal impact on leaching (Figure 8).

Zinc: leaching was minimal and was only observed during the first three intervals. Increase in lime content had marginal impact on leaching (Figure 9).

Figure 3: Cumulative mass release of chloride at varying lime content mix designs

Figure 4: Cumulative mass release of bromide at varying lime content mix designs

Figure 5: Cumulative mass release of boron at varying lime content mix designs

Figure 6: Cumulative mass release of molybdenum at varying lime content mix designs

Figure 7: Cumulative mass release of selenium at varying lime content mix designs

Figure 8: Cumulative mass release of vanadium at varying lime content mix designs

Figure 9: Cumulative mass release of zinc at varying lime content mix designs Unconfined Compression Test

The UC strengths of the monoliths are presented in Table 6. It could be observed that the compressive strength increases with curing time. The increase in compressive strength post-leaching indicated that curing continues to take place during the tank leaching test and implies potential long-term durability of the SS materials in a non-aggressive/extreme disposal environment. Increase in lime content, increased compressive strength up to 8% lime content. However, at 9% lime content, strength decreased marginally. Table 6: Unconfined compressive strength of mix-design (before and after leaching) Mix Design 7 day 14 day 28 day Post-leaching psi psi psi psi 5 % lime 131.57 410.41 422.15 866.01 6 % lime 468.55 541.69 630.39 1185.1 7 % lime 482.27 550.04 822.59 1274.44 8 % lime 67.55 557.32 700.92 2178.51 9 % lime 432.19 438.42 953.58 1473.64 CONCLUSION In this study, the fly ash mixed with FGD wastewater was stabilized with varying lime content to sequester halides and other COCs. The lime content was varied from 5 to 9%. Results from leaching Method 1315 test indicated that lime effectively sequestered SO4

2-, As, Cd, Cr, Mn, Ni, Pb, Se, Sb, and Tl. With respect to Cl-, Br-, B, Mo, Se, V, and Zn increased in lime content marginally reduced leaching. The predominant leaching mechanism of the constituents leached is diffusion, although occasional wash-off and dissolution of the elements such as Se, V, and Zn were observed. The UC strength results indicated increased compressive strength with lime content except for 9% which exhibits marginal strength decrease. The increase in compressive strength post-leaching indicated that hydration and pozzolanic reactions continue to take place during the tank leaching test, thus implying potential long-term durability of the stabilized materials in a non-aggressive/extreme disposal environment. ACKNOWLEDGEMENT The authors acknowledge the research grant from the Duke Energy and Southern Company. The support of Lhoist North America for supplying the quicklime used in this project is greatly appreciated. Finally, thanks to the numerous UNC Charlotte students who worked on this project for all their efforts and hardwork.

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